The present inventors obtained, with respect to E. coli K-12, a mutant having mutation, thrR, (herein referred to as rhtA23) that is concerned in high concentrations of threonine (>40 mg/ml) or homoserine (>5 mg/ml) in a minimal medium (Astaurova, O. B. et al., Appl. Bioch. and Microbiol., 21, 611-616 (1985)). On the basis of rhtA23 mutation an improved threonine-producing strain (SU patent No. 974817), homoserine- and glutamic acid-producing strains (Astaurova et al., Appl. Boch. And Microbiol., 27, 556-561 (1991)) were obtained.
Furthermore, the present inventors has revealed that the rhtA gene exists at 18 min on E. coli chromosome and that the rhtA gene is identical to ORF1 between pexB and ompX genes. The unit expressing a protein encoded by the ORF1 has been designated as rhtA (rht: resistance to homoserine and threonine) gene. The rhtA gene includes a 5′-noncoding region including SD sequence, ORF1 and a terminator. Also, the present inventors have found that a wild type rhtA gene participates in resistance to threonine and homoserine if cloned in a multicopy state and that enhancement of expression of the rhtA gene improves amino acid productivity of a bacterium belonging to the genus Escherichia having an ability to produce L-lysine, L-valine or L-threonine (ABSTRACTS of 17th International Congress of Biochemistry and Molecular Biology in conjugation with 1997 Annual Meeting of the American Society for Biochemistry and Molecular Biology, San Francisco, Calif. Aug. 24-29, 1997, abstract No. 457).
It is found that at least two different genes which impart homoserine resistance in a multicopy state exist in E. coli during cloning of the rhtA gene. One of the genes is the rhtA gene, however the other gene has not been elucidated.